Method of treating surfaces of rotors of the screw type rotary machine

ABSTRACT

A method treating a surface of rotors in a screw-type rotary fluid machine, with the method comprising the steps of: conducting a non-electrolytic Ni plating on the surface of each rotor so as to form a first layer of non-electrolytic Ni plating layer; heating the rotor having the first layer at a temperature not lower than 500° C.; and forming, at least, a second layer of an organic resin so as to cover the first layer.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a method of treating surfaces of rotorsof a screw-type rotary fluid machine suitable for use as a compressor,blower or a vacuum pump and to a method for treating surfaces of rotorsof a corrosion-resistant, low-noise and low-friction screw-type rotaryfluid machine which is suitable for use as a dry-type fluid machine

2. Description of the Related Art

In general, a screw-type rotary fluid machine has a male rotor and afemale rotor which are accommodated in a casing and which rotate inmeshing engagement with each other, so that a space defined by bothrotors and the casing is progressively shifted in the direction of axisof the casing while reducing its volume, thereby compressing orpressurizing a fluid confined in the space.

There are two types of screw rotary fluid machines, namely, anoil-cooled type machine in which a lubricating oil is supplied into themachine together with the fluid to be compressed so as to cool therotors and the casing while providing lubrication between the rotors andthe casing and between the meshing rotors, and a dry-type machine inwhich no oil is supplied into the machine

In the oil-cooled type machine, the male rotor and the female rotorcontact each other through the intermediary of an oil film. As the malerotor is driven by a motor through a drive gear, the female rotor isdriven by the male rotor so as to rotate in synchronization with themale rotor. Thus, the construction of the machine is comparativelysimple because no independent driving mechanism is necessary for drivingthe female rotor.

In addition, the oil supplied to the machine effectively cools therotors so as to prevent seizure of both rotors which may otherwise becaused by the friction heat generated during rotation of these rotors.

The oil-cooled type machine, however, has a in that it cannot be used inhandling fluids which are strictly required to be clean, e.g., fluidsused in food industries and high-technology industries.

In contrast, a dry-type machine can provide clean fluid because no oilis suspended in the fluid discharged from this type of machine. However,in this type of machine, both rotors rotate without contacting eachother. It is therefore necessary to employ synchronizing gears on oneend of each of the rotors so as to synchronize the rotational phases ofboth rotors. Consequently, the construction of this type of machine. iscomplicated as compared with oil-cooled type machine Another problem isthat, since both rotors do not contact each other, fluid tends to leakthrough the gap between both rotors so as to reduce compressionefficiency.

Thus, both the oil-cooled type machine and the dry-type machine haveadvantages and disadvantages.

In for example, Japanese Patent Laid-Open No. 56-75992, a screw-typefluid machine is disclosed which can be constructed both as anoil-cooled type machine and dry-type machine. This type of machine willbe referred to hereinafter as the as "first machine". In this firstmachine, the surfaces of both rotors are coated with a soft metal or aplastic so that the rotors can smoothly contact each other whilepreventing leak of fluid, thus achieving a high compression efficiency.

In Japanese Patent Laid-Open No. 58-148292 a method is disclosed inwhich a plastic, rubber or molybdenum disulfide is applied to thesurfaces of the rotor so as to reduce the gap between the rotors, thusimproving the compression efficiency. This method will be hereinafterreferred to as "first method.".

In Japanese Patent Laid-Open No. 48-2308, a dry-type machine isdisclosed in which both rotors are formed of sintered bodies and isimpregnated with an oil such that the oil is held in the micro-poresbetween particles of the sintered bodies, thereby reducing frictionbetween both rotors and, hence, wear of these rotors. This machine willhereinafter be referred to as the "second machine".

Japanese Patent Publication No. 61-47992 discloses a machine whereinsynchronizing gears, provided on the end of each of the rotors, meshwith each other so as to prevent any contact between both rollers whichare designed to rotate without contacting each other, wherein the shapesand sizes of both rotors are determined beforehand so as to avoid anoccurrence of mutual contact of the rotors due to rise of temperature ofboth rotors during operation of the machine. This machine will bereferred to hereinafter as "fourth known art," the "third machine".

In the "first machine" explained above, the surfaces of the rotors arecoated with a soft metal or a plastic. Therefore, when a corrosive fluidis handled, the soft metal or the plastic and, finally, the surfaces ofthe rotors are corroded. Thus, the "first machine" could not be usedpractically for corrosive gases.

In the "first method" described above, the surfaces of both rotors arecoated with plastic, rubber or molybdenum disulfide. As a result ofrepeated contact between both rotors during the operation of themachine, the films on both rotors are damaged and finally causeexcoriation from the rotor surfaces. In order that the requiredresistances to wear and corrosion may be provided by the plastic orrubber coating layer alone, the coating layer is required to have athickness of 0.1 mm to several centimeters. Such a thick coating layerinherently has a risk of separation from the rotor material due todifference in the thermal expansion coefficient. Thus, the second knownart is still unsatisfactory from the view point of performance,reliability and noise.

In the first machine and "first method" as described above, the surfacesof the rotors are directly treated by coating or application, and therequired resistances to wear and corrosion are provided solely by thelayer formed by the surface treatment. Consequently, the surface layeris liable to be damaged or separated in a short time after the start ofoperation of the screw-type fluid machine, causing a reduction in thecompression performance and increase in the noise level. In the worstcase, the operation of the screw-type fluid machine has to be stopped.

The "second machine" described above, a disadvantage resides in the factthe machine cannot provide clean fluid which is an essential requirementfor dry-type machines, due to the fact the rotors made of sinteredbodies are impregnated with an oil.

With regard to the "third machine" described above, mutual contactbetween the rotors during the operation is materially unavoidable dueto, for example, a machining error, even though the rotation of bothrotors are synchronized to effect non-contact rotation of both rotors.In addition, there is a risk that any foreign matter in the fluid isjammed between the rotors. If the rotor surfaces are not suitablytreated, therefore, the rotor surfaces are soon damaged to causeproblems such as a reduction in the compression efficiency andgeneration of noise.

SUMMARY OF THE INVENTION

Accordingly, a first object of the present invention is to provide amethod for treating surfaces of rotors of a screw-type rotary fluidmachine, in which noise and friction are reduced and an intimate contactbetween both rotors is increased by a suitable surface treatment of therotor surfaces.

A second object of the present invention is to provide a method fortreating the surfaces of rotors of the machine, in which the rotorsurfaces are suitably treated to positively allow mutual contact betweenboth rotors, so as to eliminate the shortcomings inherent in dry-typemachines, while keeping the advantages possessed by the dry type andoil-cooled type machines.

To achieve the first object, the present invention provides a screw-typerotary fluid machine, comprising at least one male rotor and at leastone female rotor which rotate within a casing, with the male and/orfemale rotors being surface-treated to have a treated surface structure.

The treated surface structure is preferably a multi-layered structurescomposed of a plurality of layers for improving wear resistance andcorrosion resistance.

The multi-layered treated surface structure includes a non-electrolyticplating layer and at least one covering layer or organic resin, so as tofurther improve wear resistance and corrosion resistance.

Preferably, the multi-layered treated surface structure includes anon-electrolytic Ni plating layer and at least one covering layer oforganic resin, so as to further improve wear resistance and corrosionresistance.

It is preferred that a plurality of coating layers of different organicresins are formed to reduce noise level during operation of the machine.

Preferably, the organic resin is one or more types selected from a groupof engineering plastics, so as to improve mechanical strength.

The engineering plastic is preferably one or more types selected from agroup consisting of poly sulfone resins, polyethylene terephthalateresins, polyamide resins, polyphenylene sulfide resins, polyimideresins, fluoric resins and epoxy resins.

To achieve the first object, the present invention also provides ascrew-type rotary fluid machine, comprising at least one male rotor andat least one female rotor which rotate within a casing with or withoutcontacting each other, with the male and female rotors having differenttypes of treated surface structures.

In order to improve wear resistance and corrosion resistance, one of themale and female rotors has a treated surface structure composed of anon-electrolytic Ni plating layer, while another of the rotors has atreated surface structure composed of a plurality of organic resinlayers.

To achieve the first object, the present invention also provides amethod comprising the steps of conducting a non-electrolytic Ni platingon the surface of each rotor so as to form a first layer ofnon-electrolytic Ni plating layer; heating the rotor having the firstlayer at a temperature not lower than 500° C.; and forming second layerof an organic resin or a plurality of layers of organic resins so as tocover the first layer.

To achieve the first object, the present invention also provides amethod comprising the steps of conducting a non-electrolytic Ni platingon the surface of each rotor so as to form a first layer ofnon-electrolytic Ni plating layer; heating the rotor having the firstlayer at a temperature ranging between 300° C. and 500° C.; cooling therotor to generate cracks in the non-electrolytic Ni plating layer;heating the rotor having the non-electrolytic Ni plating layer at atemperature not lower than 500° C., and forming, at least, a secondlayer of an organic resin so as to cover the first layer.

Preferably, the second layer is formed by one of injection forming andelectro-deposition with one type of resin selected from a group ofengineering plastics having high mechanical strength, and a furthercovering layer is formed to cover the second layer through injectionforming by use of one kind of resin or a mixture of two or more resinsselected from the group of the engineering plastics

In an alternative form of the method of the invention, the second layeris formed by one of injection forming and electro-deposition with amixture of resins selected from a group of engineering plastics, and afurther covering layer is formed to cover the second layer throughinjection forming by use of one kind of resin selected from the group ofengineering plastics.

To achieve the second object described before, the present inventionprovides a dry screw-type rotary fluid machine comprising at least onemale rotor and at least one female rotor which rotate within a casing,with the male and female rotors being surface-treated to havesurface-treated surface structures and being arranged to rotate incontact with each other.

In order to attain a higher adhesion between the rotor surface and thetreated surface structure, the treated surface structure is formed byapplying, to the surface of each rotor, a non-electrolytic Ni platingmaterial, and heat-treating the rotor surface carrying thenon-electrolytic Ni plating layer, thereby producing fine crystal grainstructure in the non-electrolytic Ni plating layer while allowingdiffusion of constituents at the boundary layer between the rotorsurface and the non-electrolytic Ni plating layer.

The non-electrolytic Ni plating material may be selected from a groupconsisting of Ni-P and Ni-B.

The non-electrolytic Ni plating layer may be a porous layer and may beimpregnated with a solid lubricant, in order to reduce friction withoutcausing any contamination of the fluid handled by the machine.

The solid lubricant may be selected from a group consisting of BN (boronnitride), MoS₂ (molybdenum disulfide), a fluoric resin and carbonfluoride.

To achieve a second object, the present invention also provides a methodof treating the surfaces of rotors in a dry screw-type rotary fluidmachine, comprising the steps of forming a non-electrolytic Ni platinglayer on the surface of each roll, and heat-treating the rotor surfacecarrying the non-electrolytic Ni plating layer at a temperature rangingbetween 500° C. and 650° C., thereby producing fine crystal grainstructure in the non-electrolytic Ni plating layer while allowingdiffusion of constituents at the boundary layer between the rotorsurface and the non-electrolytic Ni plating layer.

According to the first aspect of the present invention, the rotorsurface is treated to have a treated surface structure composed of aplurality of layers. More particularly, a non-electrolytic Ni platinglayer is formed as a first layer on the rotor surface, and the secondlayer or more made of organic resins are formed successively on thisfirst layer.

An experiment was conducted in which non-electrolytic Ni plating layercontaining substantially 10 wt % of phosphor (P) was formed on samplepieces of an ordinary cast iron and, after heat treatment at differenttemperatures, the numbers of cracks in the plating films on the samplepieces were measured. As will be seen from Table 1 showing the testresults, large numbers of cracks are found on samples which are treatedat temperatures between 300° C. and 500° C. However, the number of thecracks is decreased and reduced to zero as the heat-treating temperatureexceeds 500° C. This means that the plating layer exhibits superiorresistances both to wear and corrosion. That is, because of the heattreatment effected at a temperature not less than 500° C., both thetoughness and hardness of the plating film is enhanced and both Ni inthe plating film and Fe in main body metal are mutually diffusedthereinto to thereby improve the bonding between the plating film andthe rotor main body, so that the wear resistance is remarkablyincreased. In addition, the inherent cracks come to disappear in theplating film by the heat treatment effected at a temperature not morethan 500° C., so that the corrosion resistance is improved.

                  TABLE 1                                                         ______________________________________                                               200° C.                                                                      300° C.                                                                        400° C.                                                                        500° C.                                                                      550° C.                                                                      650° C.                       ______________________________________                                        Number of                                                                              0       14      4     3     0     0                                  cracks (as                                                                    observed in                                                                   region of                                                                     10 mm long)                                                                   ______________________________________                                    

On the other hand, organic resins are superior in resistance to chemicalagents, not to mention water, and are generally soft as compared withmetals. The organic resin, therefore, effectively absorb noise generatedas a result of contact between the resin layers of both rotors duringthe operation of the screw-type rotary machine. Therefore, by conductinga surface treatment for forming a multi-layered surface structure havingat least two types of layers, i.e., non-electrolytic Ni plating layerand an organic resin layer, it is possible to attain a noise reducingeffect in addition to resistances to wear and corrosion.

In one form of the invention, the heat-treatment after the Ni plating isconducted at a temperature ranging between 300° C. and 500° C. and acooling is conducted so as to generate numerous cracks in thenon-electrolytic Ni plating layer. The layer of organic resin is thenformed on the Ni-plating layer having cracks. As a result, the organicresin penetrate into the cracks so as to produce an anchoring effect,thereby attaining a high adhesion between the non-electrolytic Niplating layer and the organic resin.

The above-described surface treatment may be conducted on both the maleand female rotors. A similar effect can be obtained when theabove-described surface treatment is conducted on one of the male andfemale rotors while the other rotor is provided with a non-electrolyticNi plating layer alone.

The wear resistance can be enhanced when an engineering plastic having acomparatively high mechanical strength is used as the organic resin.

In order to reduce the difference in the thermal expansion between therotor and the organic resin, the multi-layered structure may be formedsuch that an organic resin layer closer to the rotor surface is made ofan organic resin having a smaller difference of thermal expansioncoefficient from that of the rotor material than the organic resin whichforms an organic resin layer remoter from the rotor material. It is alsopossible to reduce any stress caused by the difference in the thermalexpansion by mixing powder of metal in the organic resin layers whichform the multi-layered surface structure.

In one form of the surface treating method, a non-electrolytic Niplating is applied to the rotor material, followed by lamination withorganic resin layers formed by injection and electro-deposition,although this form of method is only illustrative.

In the second aspect of the present invention directed to a dryscrew-type rotary machine, non-electrolytic Ni plating layers formed onthe surfaces of the male and female rotors are heated at 500° C. to 650°C., so that fine crystal grain structure is formed in the plating layerso as to stabilize the plating layer. In addition, adhesion between thenon-electrolytic Ni plating layer and the rotor is remarkably improvedas a result of diffusion of constituents at the boundary between thenon-electrolytic Ni plating layer and the rotor, thus suppressing wearand separation of the non-electrolytic Ni plating layer. Furthermore,since the male rotor and the female rotor rotate in contact with eachanother, the screw-type rotary machine can exhibit superior performance.

The non-electrolytic Ni plating layer formed on the rotor may be aporous layer which is impregnated with a solid lubricant such as BN orMoS₂. In such a case, the coefficient of friction between the rotors canbe reduced to range between, for example, 0.1 and 0.2, by virtue of thepresence of the solid lubricant. The solid lubricant tends to hold itscrystalline state and, hence, hardly becomes separated powder.Therefore, the screw-type fluid machine can discharge a fluid with ahigh level of cleanliness which previously could be attained only bydry-type machines.

The above and other objects, features and advantages of the presentinvention will become clear from the following description of thepreferred embodiments when the same is read in conjunction with theaccompanying drawings.

BRIEF DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 is a longitudinal sectional view of a screw-type vacuum pump asan embodiment of the screw-type rotary fluid machine of the presentinvention;

FIG. 2 is a longitudinal sectional view of a portion of the rotor shownin FIG. 1;

FIG. 3 is a longitudinal sectional view of a dry screw-type compressoras a third embodiment of the screw-type rotary fluid machine of thepresent invention;

FIG. 4A is an enlarged sectional view of a female rotor incorporated inthe compressor shown in FIG. 3;

FIG. 4B is an enlarged sectional view of a male rotor incorporated inthe compressor shown in FIG. 3;

FIG. 4C is an enlarged sectional view of rotor surface regions encircledby broken-line circles D in FIGS. 4A and 4B;

FIG. 5 is a longitudinal sectional view of a dry screw-type vacuum pumpas a fourth embodiment of the screw-type rotary fluid machine of thepresent invention;

FIG. 6A is an enlarged sectional view of a female rotor incorporated inthe compressor shown in FIG. 5;

FIG. 6B is an enlarged sectional view of a male rotor incorporated inthe compressor shown in FIG. 5;

FIG. 6C is an enlarged sectional view of rotor surface regions encircledby broken-line circles D in FIGS. 6A and 6B;

FIG. 7 is a longitudinal sectional view of a dry screw-type rotary fluidmachine as a fifth embodiment of the screw-type rotary fluid machine ofthe present invention;

FIG. 8A is an enlarged sectional view of a female rotor incorporated inthe compressor shown in FIG. 7;

FIG. 8B is an enlarged sectional view of a male rotor incorporated inthe compressor shown in FIG. 7;

FIG. 8C is an enlarged sectional view of rotor surface regions encircledby broken-line circles D in FIGS. 7A and 7B;

FIG. 9 is a sectional schematic view of an injection molding method usedto provide a resin film on Ni-plating layer; and

FIG. 10 is a schematic view of an electrodeposition method used in theinvention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

As shown in FIG. 1 the screw-type vacuum pump has a casing 3 defining aninternal space 3a in which a male rotor 1 and a female rotor 2 mountedin a meshing relationship for rotation without contacting each other.The female rotor 2 is carried by a drive shaft 7 which is power drivenso as to drive the female rotor 2. The male rotor 1 is carried by ashaft drivingly connected to the drive shaft 7 through a pair of gears 8so that the male rotor 1 also is driven to rotate in synchronizationwith the rotation of the female rotor 1. Both the male rotor 1 and thefemale rotor 2 are made of a spheroidal graphite cast iron (FCD 55) andare surface-treated as shown in FIG. 2. More specifically, the surfaceof each of the male and female rotors 1 and 2 was treated by a processexplained below. First, there was prepared a non-electrolytic Ni-Pplating liquid containing main constituents of both NiSo₄.6H₂ O of 30g/l and NaH₂ PO₂. H₂ O of 10 g/l, sodium salt of organic acid, and anorganic acid as a reaction-accelerating agent. The rotors 1 and 2 weredipped for about 2 hours in the plating liquid having a temperature of60° C., so that a non-electrolytic Ni plating layer 4 of 20 μm inthickness was formed on each surface of the rotors 1, 2. Then, therotors 1, 2 having the Ni plating layer 4 were maintained at 300° C. forone hour under an inert gas atmosphere or under a vacuum, followed byfurnace-cooling. Consequently, there occurred cracks in the Ni-platinglayers 4. In order to enhance the toughness and hardness of theNi-plating layers 4 and to improve the adhesion between the main body ofeach of the rotors 1, 2 and the Ni-plating layer 4 by the occurrence ofmetal diffusion of Ni and Fe therebetween, the rotors 1, 2 were againheld at 550° C. one hour under an inert gas atmosphere and then cooledin a furnace. Then, a polyphenylene sulfide resin layer 5 of about 5 mmthickness was formed on the Ni plating layer 4 by use of an injectionmolding method as shown in FIG. 9, so that polyphenylene sulfide resinpenetrated into the cracks occurring in the Ni-plating layer 4 with theresult being that an anchoring effect was brought about to integrate thepolyphenylene sulfide resin 5 with the non-electrolytic Ni plating layer4. In the injection molding method, the rotor was disposed in thesplit-type metal mold 11, and the polyphenylene sulfide resin havingbeen plasticized by heating was fed under a high pressure through aninjection tube 14 into a space defined between the rotor 1 and the metalmold 11, so that the layer 5 of the polyphenylene sulfide resin wasprovided on the Ni-plating layer 4.

In the next step, by using an electrodeposition process shown in FIG.10, a coating layer 6 of an epoxy or fluoric resin having about 20 μm inthickness was adhered onto the surface of the layer 5. Morespecifically, onto the surface of the polyphenylene sulfide resin layer5, Cu was evaporated to provide Cu coating thereon, then the rotorcoated with Cu being immersed in an electrodeposition liquid 14 of about30° C. in temperature, and an electrodeposition was effected 100 secondsat 200 V by using a stainless steel anode 15 and the cathode of therotor so as to provide the layer 6. After drying the layer 6, the rotorwas held at 180° C. for 30 minutes to bake the layer 6 to therebycomplete the surface treatment.

The screw-type vacuum pump shown in FIG. 1 was formed by using the maleand female rotors 1 and 2 which were subjected to the above-describedsurface treatment. More specifically, the vacuum pump shown in FIG. 1 isa single-stage dry-type pump used to produce semi-conductors and drivenby a 2.2 kW motor and capable of discharging a gas at a rate of 100l/minute at 50 Hz to achieve a vacuum of 10⁻¹ Torr. Generally speaking,in conventional vacuum pumps of this type there occurred corrosion ofrotors after the lapse of about 6-month operation. In contrast, thevacuum pump of this invention could operate for more than twelve monthswithout any corrosion of rotors. In addition, the noise level wasreduced by about 10% as compared with conventional vacuum pumps of thiskind.

A description will now be given of a second embodiment of the presentinvention. The second embodiment incorporates rotors which weresubstantially the same as those used in the vacuum pump of the firstembodiment except that the rotors were surface-treated by a processdifferent from that of the first embodiment and in that both rotors wasmade to rotate in contact with each other.

More specifically, in the second embodiment, non-electrolytic Ni platingmaterial was applied on the rotor surfaces by use of the same conditionsas in the first embodiment and both rotors were heated at 600° C. for apredetermined time. Subsequently, a layer of a resin mixture composed ofa polyimide resin and a fluoric resin was formed by the same injectionmold method or electro-deposition as in the first embodiment on thesurface of the non-electrolytic Ni plating layer on the male rotor,while no resin layer was formed on the non-electrolytic Ni plating layeron the female rotor.

A 1.5 kW screw compressor capable of compressing a gas to 7 kgf/cm² atmaximum rotation speed of 40 m/s was assembled by using these rotors.About 10% reduction in the noise level as compared with the knowncompressor was confirmed, as well as about 15 to 20% improvement in theperformance. After a long operation both rotors showed smooth surfaceswithout any abnormal wear caused by mutual contact of rotors.

A description will now be given of a third embodiment of the inventionwith reference to FIGS. 3 to 4C.

Referring to FIGS. 3 to 4C, a later-mentioned surface treatment waseffected on the surface of the main body 1a' of a male rotor 1'.Similarly, a later-mentioned surface treatment was effected on thesurface of the main body 2a' of a female rotor 2'. The male and femalerotors 1' and 2' was made to rotate in contact with each other. Numeral3' denotes a casing.

The surface treatment was conducted in the following manner. Anon-electrolytic 90% Ni-10% P plating material 4' was applied to thesurface of each of the male rotor 1' and the female rotor 2' under thesame conditions as in the first embodiment, and the rotor was heated at600° C. for 1 hour, so that fine crystal grain structure of Ni₃ P wasformed in the non-electrolytic Ni-P plating layer 4' which had been inan amorphous state, occupying about 70 to 80% this plating layer 4', sothat the plating layer 4' was made stable. In addition, both Ni in theplating layer 4' and Fe in the main body 1' were diffused in theboundary layer 4a', so that the non-electrolytic Ni-P plating layer 4'could be strongly bonded to the boundary layer 4'a with a high adhesion.During the treatment, BN particles 9 were dispersed in thenon-electrolytic Ni-P plating layer, thus completing the surfacetreatment. The dispersion of the BN particles 9 was effected by aprocess comprising the steps of mixing BN grains into the plating liquidof the Ni-P plating which BN grains had both a columnar shape and alongitudinal length of 0.5 to 1 μm; and immersing the rotor in the BNgrains-mixed plating liquid while effecting the bubbling the platingliquid by injecting air thereinto so that the BN grains 9 was uniformlydispersed in the plating layer 4'.

The rotors 1' and 2' which were surface-treated in the described mannerwere assembled to form the dry screw-type compressor shown in FIG. 3,such that these rotors were made to rotate in contact with each other.This compressor exhibited a 15 to 20% higher improved performance ascompared with conventional non-contact dry-type screw compressors havingan efficiency in the degree of about 50% and could discharge a fluidwith a high degree of cleanness.

A description will now be given of a fourth embodiment of the inventionwith reference to FIGS. 5 to 6C.

Referring to FIGS. 5-6C, a later-mentioned surface treatment waseffected on the surface of itself 1a" of a male rotor 1". Similarly, alater-mentioned surface treatment was effected on the surface of theparts 2a" of a female rotor 2'. The male and female rotors 1" and 2"were made to rotate in contact with each other. Numeral 3" denotes acasing.

Since the surface treatment was conducted in the same manner as in thethird embodiment, the description of the surface treatment is omitted.

The rotors 1" and 2" which were surface-treated in the described mannerwere assembled to form the dry screw-type vacuum pump shown in FIG. 5,such that these rotors were made to rotate in contact with each other.This vacuum pump showed superior performance and high degree ofcleanliness of the discharged fluid as in the case of the thirdembodiment.

A description will now be given of a fifth embodiment of the inventionwith reference to FIGS. 7 to 8C.

Referring to FIGS. 7-8C, a later-mentioned surface treatment wereeffected on the surface of itself 1a' of a male rotor 1'. Similarly, alater-mentioned surface treatment has been effected on the surface ofthe parts 2a' of a female rotor 2'. The male and female rotors 1' and 2'were made to rotate in contact with each other. Numeral 3' denotes acasing.

The surface treatment was conducted in the following manner. Anon-electrolytic Ni-P plating material 4' of about 20 μm thick wasapplied, by using the same process and conditions as in the firstembodiment, to the surface of each of the male rotor 1' and the femalerotor 2', and the rotor was heated at a temperature of 300° C. to 500°C. for 1.5 hours, so that cracks 10 were generated in the surface of thenon-electrolytic Ni-P plating layer 4' at the side opposite to the mainbody 1a' or 2a' as shown in FIG. 8C. Then, the Ni-P plating layer 4' wasimpregnated with tetrafluoroethelene resin (PTFE) 11 of 10 to 20 μmthick so that PTFE filled the cracks 10, thus completing the surfacetreatment.

The rotors 1' and 2' thus surface-treated were assembled to form the dryscrew-type rotary fluid machine of FIG. 7 such that these rotors 1' and2' was made to rotate in contact with each other. Superior performanceand high degree of cleanness of discharged gas were confirmed as in thecases of the third and fourth embodiments.

As will be understood from the foregoing description, the presentinvention offers the following advantages.

According to the invention, there is provided a screw-type rotary fluidmachine comprising at least one male rotor and at least one female rotorwhich rotate within a casing, at least one of the male and female rotorsbeing surface-treated to have a treated surface structure which is amulti-layered structures composed of a plurality of layers including anon-electrolytic Ni plating player and at least one organic resincovering layer. This feature improves resistances to wear and corrosion,as well as adhesion between the layers and the main body of the rotors,while reducing noise and friction.

The invention also provides a method comprising conducting anon-electrolytic Ni plating on the surface of each rotor so as to form afirst layer of non-electrolytic Ni plating layer, heating the rotorhaving the first layer at a temperature not lower than 500° C. for apredetermined time, and forming, at least, a second layer of an organicresin so as to cover the first layer. According to this method, it ispossible to improve resistances to wear and corrosion, while reducingnoise, without allowing cracks to be generated in the first layercomposed of the non-electrolytic Ni plating layer.

The invention also provides another method which comprises conducting anon-electrolytic Ni plating on the surface of each rotor so as to form afirst layer of non-electrolytic Ni plating layer, heating the rotorhaving the first layer at a temperature ranging between 300° C. and 500°C. for a predetermined time, cooling the rotor to generate cracks in thenon-electrolytic Ni plating layer, heating the rotor having thenon-electrolytic Ni plating layer at a temperature not lower than 500°C. for a predetermined time, and forming, at least, a second layer of anorganic resin so as to cover the first layer. According to this method,it is possible to obtain a stronger adhesion between thenon-electrolytic Ni plating layer and the organic resin layer becausethe non-electrolytic Ni plating layer is impregnated with the organicresin which fills the cracks formed in the plating layer.

The present invention also provides a dry screw-type rotary fluidmachine, comprising at least one male rotor and at least one femalerotor which rotate within a casing, wherein each rotor surface isprovided with a non-electrolytic Ni-P plating layer which is treated tohave a fine crystal grain structure while allowing diffusion ofconstituents at the boundary between the plating layer and the rotormaterial. It is therefore possible to improve resistances to corrosionand wear, while ensuring high degree of cleanliness of the dischargedfluid, even though the rotors rotate in contact with each other.

When the non-electrolytic Ni plating layer is impregnated with a solidlubricant, a reduced friction and a high degree of cleanness of thedischarged fluid can be obtained simultaneously.

The invention also provides a method of treating the surfaces of rotorsin a dry screw-type rotary fluid machine having male and female rotorsrotatable in a casing, the method comprising forming a non-electrolyticNi plating layer on the surface of each roll, and heat-treating therotor surface carrying the non-electrolytic Ni plating layer at atemperature ranging between 500° C. and 650° C., thereby producing finecrystal grain structure of Ni₃ P in the non-electrolytic Ni platinglayer while allowing diffusion of constituents at the boundary layerbetween the rotor surface and the non-electrolytic Ni plating layer.According to this method, it is possible to improve the adhesion orstrength of bonding between the non-electrolytic Ni-P plating layer andthe rotor while minimizing wear and excoriation of the non-electrolyticNi plating layer.

What is claimed is:
 1. A method of treating a surface of rotors disposedin a screw rotary fluid machine having at least one male rotor and atleast one female rotor rotatable in a casing, the method comprising thesteps of: conducting a non-electrolytic Ni plating on the surface ofeach rotor so as to form a first layer of a non-electrolytic Ni platinglayer; heating said rotor having said first layer at a temperature notlower than 500° C.; and forming, at least, a second layer of an organicresin so as to cover said first layer.
 2. A method of treating a surfaceof rotors disposed in a screw rotary fluid machine having at least onemale rotor and at least one female rotor rotatable in a casing, themethod comprising the steps of: conducting a non-electrolytic Ni platingon the surface of each rotor so as to form a first layer of anon-electrolytic Ni plating layer; heating said rotor having said firstlayer at a temperature ranging between 300° C. and 500° C.; cooling saidrotor to generate cracks in said non-electrolytic Ni plating layer;heating said rotor having said non-electrolytic Ni plating layer at atemperature not lower than 500° C.; and forming, at least, a secondlayer of an organic resin so as to cover said first layer.
 3. A methodaccording to claim 1, wherein said second layer is formed by one ofinjection forming and electro-deposition with one kind of resin selectedfrom a group of engineering plastics, and a further covering layer isformed to cover said second layer by one of electro-deposition andinjection forming from one kind of resin or a mixture of two or moreresins selected from the group of engineering plastics.
 4. A methodaccording to claim 2, wherein said second layer is formed by one ofinjection forming and electro-deposition with one kind of resin selectedfrom a group of engineering plastics, and a further covering layer isformed to cover said second layer by one of electro-deposition andinjection forming from one kind of resin or a mixture of two or moreresins selected from the group of engineering plastics.
 5. A methodaccording to claim 1, wherein said second layer is formed by one ofinjection forming and electro-deposition with a mixture of resinsselected from a group of engineering plastics, and a further coveringlayer is formed to cover said second layer by one of electro-depositionand injection forming from one kind of resin selected from the group ofengineering plastics.
 6. A method according to claim 2, wherein saidsecond layer is formed by one of injection forming andelectro-deposition with a mixture of resins selected from a group ofengineering plastics, and a further covering layer is formed to coversaid second layer by one of electro-deposition and injection formingfrom one kind of resin selected from the group of engineering plastics.7. A method of treating a surface of rotors disposed in a dry screwrotary fluid machine having at least one male rotor and at least onefemale rotor which rotate within a casing in contact with each other,the method comprising: forming a non-electrolytic Ni plating layer onthe surface of each rotor, and heat-treating the rotor surface carryingsaid non-electrolytic Ni plating layer at a temperature ranging between500° C. and 650° C., thereby producing fine crystal grain structure inthe non-electrolytic Ni plating layer which allowing a diffusion ofconstituents of both the Ni plating layer and the main body of the rotorat a boundary layer between the rotor surface and said non-electrolyticNi plating layer.
 8. A method of treating a surface of a rotor disposedin a dry screw rotary fluid machine having at least one male rotor andat least one female rotor which rotate within a casing in contact witheach other, the method comprising the steps of: conducting anon-electrolytic Ni plating on the surface of each rotor so as to form anon-electrolytic Ni plating layer; heating said rotor having said firstlayer at a temperature ranging between 300° C. and 500° C.; causingcracks to be generated in said non-electrolytic Ni plating layer;heating said rotor having said non-electrolytic Ni plating layer at atemperature not lower than 500° C., and impregnating saidnon-electrolytic Ni plating layer with a solid lubricant.